How Effective Are LOI's (Letters of Introduction)?

We freelancers land work in a variety of ways. Most of us secure assignments through the old-fashioned query. A few lucky ones have established relationships with editors in which the editors come to them. A third option is to send a letter of introduction (LOI) to editors, which basically tests the waters of a publication before you send out a formal query.

An LOI is great if you aren't sure the publication even uses freelancers. There's nothing worse than wasting your efforts putting together a killer query only to have it rejected because you didn't do a little bit of extra research. An LOI can help you save face.

Personally, I've sent out a few LOI's over the years, with mixed results. I've been on a hot streak with querying lately and sent out a few LOI's for kicks, too. I send it directly to the editor if there's one listed on the website or masthead, but I've also sent them to the generic "editor@", "info@", or "mail@" addresses.

I've sent LOI's as basic as:

Dear [Editor's Name]:
I'm a freelance writer and I would be very interested in contributing to your publication/website. Do you typically work with freelancers? Could I receive a copy of your writers' guidelines?
My work has appeared in blah blah blah. I'm very interested in contributing to your publication based on my interest in [insert subject area here].
Thank you for your consideration. I look forward to hearing from you.
Sincerely,
Sara Hodon

Of the 10 or so LOI's I've sent out recently, I've only gotten a response from about 2 editors. Not great odds in my eyes. But I'll keep sending them. 'Cause that's what I do.

What about you? Have you had luck with the LOI's, or do you just take your chances and go direct to the query?